128 THE ANATOMY OF WOODY PLANTS 



in the elements of the simple epidermis. In Ficus elastica, on the 

 other hand, the multiplication of the epidermal layer has resulted 

 in the imbedding of the cystoliths deeply in the substance of the 

 water tissue. Cystoliths are a valuable diagnostic feature in the 

 case of the Urticaceae and Acanthaceae. 



In many gymnospenns the epidermal layer is reinforced, 

 frequently in the leaf and more rarely and only in the most 

 ancient representatives of the group in the stem, by a layer of 

 colorless and generally strongly thickened cells known as the hy- 

 poderma. It is not at the present time clear that the hypoderma 

 so called in gymnospermous leaves is derived from the epidermis 

 by the multiplication of its cells, but the structure in question is 

 most conveniently discussed under this head. This layer occurs 

 very distinctly in the leaves of the cycads, Ginkgo, and the conifers, 

 and deepens the strata of colorless cells bounding the surfaces of the 

 leaf. In older types the hypodermal layers tend to become ribbed, 

 and in the leaves of Mesozoic pines and other conifers, as well as of 

 the Cordai tales of the Paleozoic, this condition is often very marked. 

 In the Cordaitales, Cycadofilicales, and associated groups the ribbed 

 condition of the tissues supporting the epidermis is often strongly 

 developed in the stem organs. It is not, however, by any means 

 settled whether the hypodermal layers are of common origin with 

 the epidermis and hence belong to the same histological category. 



In plants exposed to extreme drought, physical or physiological, 

 the epidermis responds not only by the frequent multiplication of 

 its layers but also by the development of a well-marked cuticle and 

 by the cuticularization of the outer region of the external walls of its 

 cells. This condition is well developed in plants of desert habit 

 or in those rooted in poisonous soil, even when moist conditions are 

 present. In aquatics, on the other hand, the cuticular structures 

 are poorly differentiated and may be lacking even in the case of 

 the spores, which more constantly than any other structures of the 

 higher plants maintain a cuticularized exine or outer coat. The 

 question of the development of cuticle in plants is of some interest 

 from the standpoint of past climatic conditions on the surface of 

 our earth, although even here it is not conclusive on account of the 

 nearly identical influences of dry and merely poisonous substrata. 



